The present invention relates generally to the wet process for the recovery of uranium from phosphoric acid which utilizes a solution comprising di(2-ethylhexyl) phosphoric acid (hereinafter referred to as DEPA) and trioctylphosphine oxide (hereinafter referred to as TOPO) dissolved in an organic diluent, and more specifically, it relates to the reductive stripping of uranium from the aforesaid solution.
There are five uranium recovery plants in the United States that are currently recovering about 1,500 tons of U.sub.3 O.sub.8 per year by the DEPA-TOPO process disclosed by Fred J. Hurst in U.S. Pat. No. 3,711,591. Three additional plants, two of which used other recovery processes, have ceased operation because of technical difficulties coupled with adverse economic conditions. The operating plants have reported a number of unexpected operating problems, most of which are associated with the reductive strip step.
Typically, the reductive strip system consists of three counter-current mixer-settler stages operated at about 50.degree. C. The DEPA-TOPO extract containing about 0.3 g U (VI)/1 (0.0015 molar) is contacted in the first mixer with a very small volume (1/35 to 1/50 of the solvent flow) of advancing 5 to 6 molar phosphoric acid strip solution containing Fe(II) in order to reductively strip the uranium and concentrate it to 10-15 g/l . The large disparity in flow rates dictates a large internal recycle of the aqueous phase (2/1 ratio of acid to organic using 6 molar H.sub.3 PO.sub.4) from each settler to each mixer in order to achieve efficient mixing of the solutions and stripping of the uranium. Currently, most plants are operating with a recycle ratio of acid to organic of 1/2 or 1/3. The use of this significant reduction in recycle ratio, coupled with the lower acid concentration (5 to 5.5 molar) currently being produced by the phosphate industry, is suspected of being responsible for the difficulties being experienced, and has indicated a need for re-examination of the basic chemistry and mechanisms involved in the reductive strip step of the process.